Japanese Laid-Open Patent Publication No. 2009-106136 discloses a charging system in which a vehicle mounted electrical storage device is charged by a power source outside the vehicle through wireless reception of charging power through the resonance method. Specifically, the charging system of the above document includes an electric vehicle and a power supply device. The electric vehicle has a secondary self-resonance coil, which is a secondary-side resonance coil, a secondary coil, a rectifier, and an electrical storage device. The power supply device has a high-frequency power driver, a primary coil, and a primary self-resonance coil, which is a primary-side resonance coil. The number of turns of the secondary self-resonance coil is determined based on the voltage of the electrical storage device, the distance between the primary self-resonance coil and the secondary self-resonance coil, and the resonant frequency of the primary self-resonance coil and the secondary self-resonance coil. The distance between the power supply device and the vehicle changes depending on the conditions of the vehicle, for example, the loading state and the tire air pressure. Changes in the distance between the primary self-resonance coil of the power supply device and the secondary self-resonance coil of the vehicle change the resonant frequency of the primary self-resonance coil and the secondary self-resonance coil. Therefore, according to the above document, a variable capacitor is connected between the ends of the wire forming the secondary self-resonance coil. When charging an electrical storage device, the charging system calculates the charging power of the electrical storage device based on detection values of a voltage sensor and a current sensor, and adjusts the capacitance of the variable capacitor of the secondary self-resonance coil such that the charging power is maximized. The above document discloses that the LC resonant frequency of the secondary self-resonance coil is adjusted in this manner.
Japanese Laid-Open Patent Publication No. 2008-120357 discloses a non-contact power supply device for a moving body. The non-contact power supply device supplies power, without contact, to a power receiving portion of a moving body from a power supply portion through electromagnetic induction. The power supply portion of the non-contact power supply device is installed on the surface of a road on which the moving body travels. The power receiving portion is provided at such a position that, when the moving body is stopped at the position where the power supply portion is installed, the power receiving portion faces the power supply portion with a predetermined space in between. The non-contact power supply device has a simple movable device for moving the power receiving portion vertically. This allows the distance between the power receiving portion and the power supply portion to be finely adjusted.
Japanese Laid-Open Patent Publication No. 2009-106136 discloses a method described below for efficiently supplying power from the power supplying side to the power receiving side. That is, even when the distance between the primary self-resonance coil and the secondary self-resonance coil changes due to the condition of the vehicle, for example, the loading state or the tire air pressure, the capacitance of the variable capacitor of the secondary self-resonance coil is adjusted such that the charging power of the electrical storage device is maximized when the electrical storage device is charged. However, according to this power supplying method, the impedance is adjusted only the variable capacitor. Therefore, if the required adjustment is large, the size of the variable capacitor has to be increased.
As described above, the non-contact power supply device disclosed in Japanese Laid-Open Patent Publication No. 2008-120357 supplies power without contact through electromagnetic induction. In the case of electromagnetic induction, a power supply portion and a power receiving portion can be moved simply closer to each other. However, in the case of a resonance type non-contact power supply, the distance between a power supply portion and a power receiving portion needs to be adequate. In other words, in the case of the resonance type non-contact power supply, the distance between the power supply portion and the power receiving portion cannot be positioned at an adequate distance simply by bringing the power supply portion and the power receiving portion close to each other.